The present invention relates to an optical-use adhesive film and a biaxially oriented polyester film. More particularly, the invention relates to a biaxially oriented adhesive polyester film well suited for optical applications such as a base film of a prism lens sheet (Brightness Enhance Film) in a LCD, base films for a hard coat processed film and a base film for an AR(anti-reflection) film, and a protective film in a CRT.
A biaxially oriented polyester film has widely been used as various kinds of optical-use films because of the superior transparency, dimensional stability and chemical resistance.
Especially, in a case where the biaxially oriented polyester film is adopted as a base film for a prism lens sheet used in LCD, base films for hard coat processed film and an anti-reflection film, and a protective film used for CRT, the polyester film having a relatively large thickness of 50 xcexcm or more has been preferred in order to meet requirements for superior mechanical strength and dimensional stability. Generally such an optical-use film has been encountered by requirements not only for superior adhesiveness suitable for prism lens processing, hard coat processing and anti-reflection processing, which are post-processing, but for extreme scarcity of fine foreign matters included in a film that is a cause for optical defects. Another requirement remains for a superior transparency.
Generally, a biaxially oriented polyester film, however, has been known its poor adhesiveness to other materials such as an acryl-based resin constituting a prism lens and a hard coat layer as a main component. Hence, a proposal has been presented in JP 94-340049, for example, in which an adhesive layer made from polyurethane resin or the like is laminated on a polyester. However, a polyester film on which an adhesive layer made from polyurethane resin is laminated can be improved in adhesion to an outer layer such as a hard coat, whereas the polyurethane layer has no sufficient adhesion to a polyester film as a base, which eventually leads to a problem of insufficient adhesion to the outer layer.
Further, in order to improve a slip property of the film, there has generally been adopted a technique in which surface irregularities such as peaks and valleys are imparted on a film surface by introducing particles into the film, but the particles in the film is a cause for poor transparency thereof. Accordingly, in order to attain a highly transparent film, it is desired that a content of the particles, which are incorporated to impart a slip property to the base film, is at a low level, or no particles are contained.
Further, a refractive index of a coating layer used for imparting adhesive also produces a problem since a light transmittance changes by interference between lights respectively reflected from front and rear surfaces of a film: a coating layer with superior adhesiveness decreases a light transmittance of a film applied with coating layer.
In addition, a conventional polyester film has an irregularity in thermal shrinking ratio over a surface thereof: a thermal shrinking ratio along a longitudinal (machine) direction is different according to a measuring position along a transverse direction. Therefore, when the polyester film is used for an optical-use film, lengths of both side ends of the polyester film differently change from each other due to transverse unevenness of longitudinal thermal shrinking ratio in a heat treatment step of post-processing such as prism lens processing, hard coat processing and anti-reflection processing, thereby a problem has been arisen. The reason why the problem has arisen is such that, for example, both side ends of such the polyester film are rubbed by a frame of a machine base or the like in the heat treatment step due to a difference in the length change between both sides when the film passes through the machine, with the result that the side ends receive some damages: a pass-through property of the film is worsened in a post-processing step, further leading to reduction of its productivity in a post-processing step. Especially, development in large-sized display panel such as of a note-type personal computer and a CRT has rapidly progressed in recent years, whereby the problem will be more and more serious when in wider product application.
Further, when a conventional optical-use adhesive film with a biaxially oriented base film of polyethylene terephthalate is subjected to a heat treatment step at a temperature in the range of from 100 to 150xc2x0 C. in post-processing such as prism lens processing and hard coat processing, a problem has arisen since a haze value of film increases and a white appearance defect is generated. The problem leads direct to low visibility and reduced quality, improvement of which has been desired.
Still further, a problem has arisen since warpage arises by shrinkage of a polyester film in a heat treatment of the post-processing step and not only is a pass-through property deteriorated but reduction in productivity occurs. Another problem has also arisen since such a deterioration of the pass-through property is a hindrance in assembly of a liquid crystal display apparatus. Such a warpage of a film caused by a heat treatment accompanying the post-processing step has been demanded to be the smallest possible.
As described above, the optical-use film has been requested so as to have adaptability in the post-processing step such as prism lens processing, hard coat processing and anti-reflection processing.
In light of the above-described problems, it is an object of the invention to provide an optical-use adhesive film that has adhesive and superior transparency in an optical-use film application in which an adhesive layer is necessary on at least one surface of a film and which has a small thickness variation ratio.
It is another object of the invention to improve an adaptability of the optical-use adhesive film in a post-processing step such as prism lens processing, hard coat processing or anti-reflection processing. More particularly, it is the object of the invention to provide the optical-use adhesive film whose transportability can be improved in a thermal treatment of a post-processing step, increase in whose haze value is small and whose warpage is small.
The present inventors have conducted serious studies in order to achieve the objects and discovered that the objects of the present invention can be achieved by the following means:
1. An optical-use adhesive film, comprising a base film of the optical-use adhesive film is a biaxially oriented polyester film having a thickness equal to or more than 50 xcexcm, and an adhesive polymer layer laminated on at least one surface of the base film, wherein refractive indexes of the base film in longitudinal and transverse directions are both in the range of from 1.55 to 1.70 and that refractive indexes of the adhesive layer and the base film satisfies the following formula (1):
(Nxxe2x88x92Nc)2+(Nyxe2x88x92Nc)2xe2x89xa60.0300 xe2x80x83xe2x80x83(1), 
wherein Nx and Ny respectively indicate refractive indexes in longitudinal and transverse directions of a base film and Nc is a refractive index of an adhesive layer.
2. An optical-use adhesive film according to claim 1, wherein a thickness of the base film is in the range of from 100 to 300 xcexcm.
3. An optical-use adhesive film according to claim 1, wherein the biaxially oriented polyester film is a polyester film made from polyethylene terephthalate as a main component.
4. An optical-use adhesive film according to claim 1, wherein the polymer adhesive layer includes a copolymerized polyester-based resin and a polyurethane-based resin.
5. An optical-use adhesive film according to claim 4, wherein the copolymerized polyester-based resin has a branched monomer as a constituent.
6. An optical-use adhesive film according to claim 1, wherein the polymer adhesive layer includes particles.
7. An optical-use adhesive film according to claim 6, wherein the adhesive layer include particles having an average size in the range of from 20 nm to 300 nm, only the lower limit being included, and particles of an average size in the range of from 300 nm to 1000 nm, both limits being included.
8. An optical-use adhesive film according to claim 1, wherein a haze value of the optical-use adhesive film is equal to or less than 1.0%.
9. An optical-use adhesive film according to claim 1, wherein that a thickness variation ratio is equal to or less than 3%.
10. An optical-use adhesive film according to claim 1, wherein a difference between the maximum value of a longitudinal thermal shrinking ratio and the minimum value thereof along a transverse direction of the optical-use adhesive film is equal to or less than 0.2%.
11. An optical-use adhesive film according to claim 1, wherein an adhesiveness of the optical-use adhesive film when a photo-curable acryl-based coating layer is laminated on an adhesive layer is equal to or higher than 85% in adhesiveness obtained from the following formula (2) whose valuables on its left side are determined by a testing method in conformance with description in a section 8.5.1 of JIS-K5400;
Adhesiveness (%)=(1xe2x80x94peel area/test area)xc3x97100 xe2x80x83xe2x80x83(2). 
12. An optical-use adhesive film according to claim 1, wherein a total light transmittance of the optical-use adhesive film is equal to or higher than 90% and an increase in haze value after the optical-use adhesive film receives a heat treatment at 150xc2x0 C. for 120 min is equal to or less than 0.5%.
13. An optical-use adhesive film according to claim 1, wherein a content of cyclic trimers in the base film is equal to or less than 5000 ppm.
14. An optical-use adhesive film according to claim 1, wherein a total light transmittance of the optical-use adhesive film is equal to or higher than 90% and a warpage of the optical-use adhesive film in a specimen size 300 mmxc3x97210 mm after the optical-use adhesive film receives a heat treatment at 150xc2x0 C. for 3 hr is equal to or less than 3 mm.
15. A biaxially oriented polyester film ,comprising protrusions are formed, by an emboss-processing roll, on at least one surface of the biaxially oriented polyester film in the neighborhoods of both ends in a transverse direction thereof, wherein a ratio of a sum of vertical projection areas on a film surface of the biaxially oriented polyester film of portions thereof deformed by emboss processing, to a total area of the biaxially oriented polyester film with which the emboss-processing roll is put into contact is in the range of from 0.01 to 0.25.
16. An optical-use adhesive film according to claim 1, wherein protrusions are formed, by an emboss-processing roll, on at least one surface of the optical-use adhesive film in the neighborhoods of both ends in a transverse direction thereof, wherein a ratio of a sum of vertical projection areas on a film surface of the optical-use adhesive film of portions thereof deformed by emboss processing, to a total area of the optical-use adhesive film with which the emboss-processing roll is put into contact is in the range of from 0.01 to 0.25.
17. An optical-use adhesive film according to claim 16, wherein a vertical projection area on a film surface of the optical-use adhesive film of any one portion thereof deformed by emboss processing is in the range of from 0.01 to 0.2 mm2.
18. An optical-use adhesive film according to claim 16, wherein a height of a portion of the optical-use adhesive film deformed by emboss-processing is in the range of from 1 to 20% of an average thickness of a non-processed region thereof.
19. An optical-use adhesive film according to claim 16, wherein an average thickness of a non-processed region of the optical-use adhesive film is in the range of from 30 to 300 xcexcm and an average height of portions of the optical-use adhesive film deformed by emboss-processing is in the range of from 3 to 30 xcexcm.
20. An optical-use adhesive film according to claim 16, wherein a width of a region that is emboss-processed in the optical-use adhesive film is in the range of from 5 to 20 mm.
21. A roll that is obtained by take-up a biaxially oriented polyester film according to claim 15.
22. A roll that is obtained by take-up an optical-use adhesive film according to claim 16.